1. Field of the Invention
The present invention relates to a method for transmitting data through a wireless local area network (WLAN), an access point device and a station device.
2. Description of the Related Art
WLANs with high speeds of over 100 Mbits/sec are now required in line with the wide penetration and development of digital products. A multiple-input-multiple-output (MIMO) technology using multiple transmission and reception antennas is recognized as one of the candidates that can satisfy this development demand. The multiple transmission and reception antennas technology includes a technique enabling high speed data transmission without increasing the bandwidth of a system by transmitting different data units at the same time by using multiple antennas for a transmitter and a receiver, and a technique obtaining transmission diversity by transmitting the same data to multiple transmission antennas. The MIMO technology is one of the adaptive array antenna technologies using a plurality of antennas to electrically control directivity, and narrows the directivity on a beam to form a plurality of independent transmission channels and increases the transmission speed as many times as the number of antennas. At this time, the frequency and transmission timing used by each antenna is identical. Also, by using multiple channels, due to the development of the technology enabling a bandwidth twice or more wider than the existing bandwidth, the transmission capability has been greatly increased.
Meanwhile, the IEEE 802.11a/g standard defines three separate 20 MHz channels and the orthogonal frequency division multiplexing (OFDM). Unlike the IEEE 802.11a/g standard implementing three separate 20 MHz wireless carrier channels, WLAN using channel bonding combines two of the carrier channels into one 40 MHz channel in order to increase the throughput. That is, the WLAN channel bonding is based on two neighboring IEEE 802.11 and OFDM channels to achieve a throughput of a large amount of data. The channel bonding doubles the size of a fast Fourier transform (FFT) and enables the FFT to multiplex twice the data amount. Except that 128 point FFT is implemented, the channel bonding follows all the processings of the original IEEE 802.11a/g standard. Also, in order to maintain a standard 802.11 symbol interval through a 40 MHz channel that is a bandwidth twice wider than that of the standard 802.11 channel, the sampling and clock rates should be doubled.
Thus, with the introduction of the MIMO technology and the channel bonding technology, legacy stations complying with the existing 802.11 standard and stations following the introduction of the new technologies can be mixedly disposed in one basic service set (BSS).
FIG. 1 is a diagram showing a BSS status in which single-input-single-output (SISO) stations and MIMO stations are mixedly disposed according to the related art technology.
Referring to FIG. 1, the BSS can include two SISO stations 110 and 120, two MIMO stations 130 and 140, and two channel bonding stations 150 and 160. The SISO station represents a legacy station complying with the conventional IEEE 802.11 standard, and the MIMO station and the channel bonding station can be regarded as high throughput stations.
Meanwhile, in a high speed WLAN based on the IEEE 802.11a/g standard, a media access control (MAC) mechanism is formed with a distributed coordination function (DCF) period in which a plurality of stations are trying to access channels in a carrier sense multiple access/collision avoidance (CSMA/CA) method. Also, in order to reduce a collision probability, a binary random backoff is performed, and a point coordination function (PCF) period in which the order of transmission of data by each station is allocated through polling scheduling centralized by an access point (AP) with an embedded point coordinator (PC). Also, by using the OFDM, a maximum data rate of 54 Mbps is supported and by using forwarding equivalence classes (FEC), a higher restoration ratio of damaged data is guaranteed.
In an IEEE 802.11 WLAN, a wireless medium is shared, and communication is performed between stations. Access to this wireless medium is controlled through a “coordination function” in a LAN module. The IEEE 802.11 WLAN supports two coordination functions, the DCF and the PCF. That is, as shown in FIG. 2, the IEEE 802.11 WLAN has a form in which the PCF 220 operates on the DCF 210.
Referring to FIG. 3, the PCF and the DCF will now be explained.
The DCF uses the CSMA/CA that is a mechanism similar to the carrier sense multiple access/collision detection (CSMA/CD), which is an access method used in the IEEE 802.3 WLAN. The PCF uses a method in which a special station referred to as a point coordinator (PC) controls medium access in a centralized method. The DCF is a contention-based service, performing a backoff mechanism in order to effectively share a given channel while preventing collision between stations to the greatest degree. In the PCF, in a contention free period (CFP) 300, a PC allocates a channel use right to stations, in order, not through channel contention, but through polling. Then, if a contention period (CP) begins, a use right is obtained through channel contention with backoff again. The CFP begins by the PC broadcasting a beacon frame 302 and ends by transmitting a CF-End frame 311. In this beacon frame, a value referred to as a network allocation vector (NAV) 330 is included. The NAV plays a role to make stations that are participating in a current network and desire to use a channel follow the control of the PC, by making the stations temporarily stop independent operations during only the CFP period. If the CFP period ends, a CP operates again according to the DCF rule.
Referring to FIG. 3, the operation of the IEEE 802.11 PCF will now be explained in more detail.
A PC broadcasts a beacon frame 302 so as to inform all stations under control of the PC of a CFP period 300.
The stations receiving the beacon 302 stop all individual operations and only a station having an address specified in a poll frame transmitted by the PC after the beacon frame has a channel access right and is enabled to transmit data.
The CFP period 300 means that the DCF function in which a channel access right is obtained through contention is temporarily stopped and a mechanism for channel access through polling by a PC begins.
After the beacon frame, the PC performs polling in order based on a predefined polling list. If there is data to be transmitted to a station of which turn it is for polling, the PC loads data on a polling frame 304 and transmits to the station, and if the PC has no data to transmit, only the polling frame 304 is transmitted to the station so that the station can have an opportunity to transmit data. Then, the station receiving this polling frame transmits an ACK frame 306, as a reception confirmation response, to the PC, after a short inter-frame space (SIFS) 305 that is a time to prepare a response elapses. As in the polling frame, if there is data to transmit, the station loads the data in the ACK frame 306 and transmits the data, and if there is no data to transmit, the station transmits only the ACK frame 306 to the PC.
Based on the polling list, the PC repeats this process with stations registered in the polling list.
If the CFP ends or if all the stations in the polling list are polled once before the CFP ends, the PC broadcasts CF-End frame 311 to return the control right held by the PC till that time, to all stations such that channel contention can be started.
However, a case where different systems are mixedly disposed as in the BSS shown in FIG. 1, that is, a BSS of a WLAN where legacy stations complying with the existing 802.11 standard and high throughput stations (for example, MIMO stations or SISO stations using the channel bonding technology) are coexisting can be thought of. In this case, since in the related art legacy stations, a transmission frame of a high throughput station cannot be recognized, there is a possibility of collision because of communication by the legacy stations during transmission by high throughput stations. Accordingly, in the related art technology using the structure in which a channel access right is obtained through contention, the channel access right cannot be guaranteed for high throughput stations in a system where legacy stations and high throughput stations are mixedly disposed.